One of the basic properties of a photographic light-sensitive material comprising a silver halide emulsion coated on a support is that the developed silver density varies in relation to the change in exposure, i.e., the property of gradation. This property is important particularly where it is necessary to recognize fine defects, e.g., in the case of X-ray photography. Examples of a process for improving gradation include a method as disclosed in U.S. Pat. No. 3,574,628 which comprises use of a monodisperse emulsion or a development fog inhibitor such as an azole. However, an emulsion which has been subjected to such a process to adjust gradation is disadvantageous in that when it is subjected to X-ray exposure or high intensity exposure using a screen other than a fluorescent screen, the gradation thereof itself is lowered.
X-ray direct exposure is used for dental X-ray film, mammographic X-ray film, and industrial X-ray film. For high intensity exposure, laser exposure and the like may be used.
Particularly, the gradation property in an industrial X-ray film has much to do with the degree of defect recognition. For example, in the case of detection of defects in a heavy metal system, increasing the film contrast is the only way to improve the recognition degree, because the contrast of the imaged object is too low.
It has also been known that the iodine content may be increased to improve the efficiency of absorption of radiation. However, a high iodine content silver halide grain normally has a low developability, making it difficult to provide an emulsion having a high gradation.
On the other hand, JP-A-53-22408 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application"), JP-B-43-13162 (the term "JP-B" as used herein refers to an "examined Japanese patent publication"), and J. Photo. Sci., 24, 198 (1976) describe that a laminated type silver halide grain comprising a core covered with a plurality of shells may be used to improve developability or provide a higher sensitivity.
However, silver halide grains prepared for these purposes do not always give an improvement in gradation under X-ray exposure or high intensity exposure. For example, JP-A-53-22408 describes a laminated type silver halide grain comprising a pure silver bromide (core)/silver iodobromide (iodine content: 1 mol %)/pure silver bromide structure. This silver halide grain provides a lower gradation at X-ray exposure or high intensity exposure rather than improving the gradation.
Silver halide grains comprising a coating layer obtained by halogen substitution are described in West German Patent 2,932,650, and JP-A-51-2417, JP-A-51-17436, and JP-A-52-11927. However, these silver halide grains are disadvantageous in that even though they may improve the fixing rate, they may cause development inhibition, making it impossible to provide a desired sensitivity. Therefore, these silver halide grains cannot be put into practical use to provide a negative emulsion having desired gradation.
Positive (internal latent image type) silver halide grains comprising a core covered with a plurality of coating layers by halogen substitution have been known. Such positive silver halide grains are described in detail in U.S. Pat. Nos. 2,592,250 and 4,075,020 and JP-A-55-l27549. These silver halide grains are often used for diffusion transfer internal latent image type direct positive light-sensitive materials. However, since these silver halide grains have too high an internal sensitivity, they can never be used for negative emulsions suitable for exposure of the type to which the present invention is applied.
An approach which comprises sensitizing the surface of silver halide grains is described in West German Patent 2,932,650. However, even such silver halide grains cannot provide an gradation at X-ray exposure or high intensity exposure.
JP-A-55-l27549 describes a silver halide emulsion containing grains comprising a core containing almost 100% silver iodide, covered with silver iodobromide. Such a core composition i obtained by replacing chlorine with bromine and bromine with iodine. However, such a silver halide emulsion is disadvantageous in that it is very susceptible to pressure desensitization, making it unsuitable for practical use. Even if such a silver halide emulsion is sensitized on the surface of the grains so that it is converted to a negative emulsion, it is still subject to pressure desensitization and cannot provide in improved gradation, making it unsuitable for practical use.